Recombinant Mouse CD99 Fc Chimera Protein, CF

Mouse CD99, also called PILR-L (paired immunoglobulin-like type 2 receptor ligand), is a 25 kDa type I transmembrane glycoprotein. The CD99 family of molecules also includes CD99-L2, Xga, and the pseudogene CD99L1 (1 - 4). The mouse CD99 cDNA encodes a 175 amino acid (aa) precursor with a 28 aa signal sequence, a 109 aa extracellular domain (ECD), a 21 aa transmembrane segment, and a 17 aa cytoplasmic region. The ECD contains no identifiable motifs, N-linked glycosylation sites, or cysteine residues, but has acidic, proline- and glycine-rich regions and sites for O-linked glycosylation that are conserved across species (4, 5). Mouse CD99 is thought to be rapidly evolving, and shares only 60% aa identity with rat, plus 40 - 48% aa identity with human, equine and canine CD99 (3, 5). The only known isoform of mouse CD99 appears to have a non-functional signal sequence (5). In contrast, human CD99 has long and short isoforms that show differences in function (6). Mouse CD99 may be either expressed on the surface, or retained within the cell as a homodimer, as found in naive CD4⁺ or CD8⁺ T cells (5, 7). CD99 is expressed by most leukocytes and vascular endothelial cells, especially at contact sites (3, 8). CD99 molecules on one cell can interact with CD99 on adjacent cells. CD99 can also interact with PILR-alpha and PILR-beta, which are expressed on granulocytes, macrophages, NK and dendritic cells (1, 9). The interactions regulate the transendothelial migration of lymphocytes and neutrophils to inflamed tissues (9). Sialylated sugars O-linked to CD99 at Thr45 and Thr50 mediate PILR recognition of CD99. Either is sufficient for inhibitory PILR-alpha recognition, while the Thr45 site is specifically required for activating PILR-beta recognition (10, 11).